This invention relates to solid inkjet printheads. In inkjet printing, a printhead is provided, the printhead having at least one ink-filled channel for communication with an ink supply chamber at one end of the ink-filled channel. An opposite end of the ink-filled channel has a nozzle opening from which droplets of ink are ejected onto a recording medium. In accordance with the ink droplet ejection, the printhead forms an image on the recording medium. The ink droplets are formed as ink forms a meniscus at each nozzle opening prior to being ejected from the printhead. After a droplet is ejected, additional ink surges to the nozzle opening to reform the meniscus.
The direction of the ink jet determines the accuracy of placement of the droplet on the receptor medium, which, in turn, determines the quality of printing performed by the printer. Accordingly, precise jet directionality is an important property of a high quality printhead. Precise jet directionality ensures that ink droplets will be placed precisely where desired on the printed document. Poor jet directionality results in the generation of deformed characters and visually objectionable banding in half tone pictorial images. Particularly with the newer generation of thermal ink jet printers having higher resolution enabling printing at least 360 dots per inch, improved print quality is demanded by customers.
A major source of ink jet misdirection is associated with improper wetting of the front face of the printhead containing at least one nozzle opening. One factor that adversely affects jet directional accuracy is the accumulation of dirt and debris, including paper fibers, on the front face of the printhead. Another factor that adversely affects jet directional accuracy is the interaction of ink previously accumulated on the front face of the printhead with the exiting droplets. This accumulation is a direct consequence of the forces of surface tension, the accumulation becoming progressively severe with aging due to chemical degradation (including, for example, oxidation, hydrolysis, reduction (of fluorine), etc.) of the front face of the printhead. Ink may accumulate on the printhead front face due to either overflow during the refill surge of ink or the splatter of small droplets resulting from the process of ejecting droplets from the printhead. When accumulated ink on the front face of the printhead makes contact with ink in the channel (and in particular with the ink meniscus at the nozzle orifice), the meniscus distorts, resulting in an imbalance of forces acting on the ejected droplet. This distortion leads to ink jet misdirection. This wetting phenomenon becomes more troublesome after extensive use of the printhead as the front face either chemically degrades or becomes covered with dried ink film. As a result, gradual deterioration of the generated image quality occurs.
One way of avoiding these problems is to control the wetting characteristics of the printhead front face so that no accumulation of ink occurs on the front face even after extensive printing. Thus, in order to provide accurate ink jet directionality, wetting of the front face of the printhead is preferably suppressed. This can be achieved by rendering the printhead front face hydrophobic.
Conventionally, a solid inkjet printhead has been built with stainless steel plates etched chemically or punched mechanically. A solid printhead has also been built on a silicon substrate with microelectro-mechanical system (MEMS) technology. There has been significant effort recently to reduce the cost of solid inkjet printheads. One opportunity is to replace the stainless steel aperture plate with a polyimide aperture plate. For stainless steel material, the aperture was punched mechanically. Therefore, by replacing it with a polyimide film that can be laser cut, it is possible to eliminate issues with defects and limitations in the punched stainless steel. In addition, a polyimide aperture plate significantly reduces manufacturing costs as compared to the punched stainless steel plate. The hole size and size distribution are comparable to stainless steel aperture plates in a polyimide plate.
Polyimide is used in many electronic applications for its many advantages, such as high strength, heat resistant, stiffness and dimensional stability. In solid inkjet printheads, it can be used as an aperture plate for ink nozzles. However, without an anti-wetting or hydrophobic coating, the front face will flood with ink and the jetting cannot be done. But the high surface energy nature of the polymer can cause some issues. Therefore, protective coatings with low surface energy characteristics are key to long lasting devices.
For example, U.S. Pat. No. 5,218,381, incorporated herein by reference in its entirety, describes a coating comprising an epoxy adhesive resin such as EPON 1001F, for example, doped with a silicone rubber compound such as RTV 732. The coating can be provided in the form of a 24% solution of EPON 1001F and a 30:70 mixture of xylene and methyl iso-butyl ketone by weight doped with 1% by weight of RTV 732. Such a coating enables the directionality of an ink jet to be maintained for the printing lifetime of the printer. An adhesion promoter such as a silane component, for example, can also be included to provide a highly adherent, long lasting coating.
While laser ablated nozzle plates are able to provide excellent drop ejector performance, a practical problem in so forming the nozzle plates is that while polymer materials used for the nozzle plate, for example polyimides, are laser abatable with lasers such as excimer lasers, such polymers are not hydrophobic. It is thus necessary to provide a hydrophobic coating upon the surface of the nozzle plate to render the front face hydrophobic to improve the ink jet accuracy as discussed above. However, coating polyimide is not commonly done in industry. Polyimide is chemically and thermally stable, and many coating agents cannot easily form a thin and uniform coating on the surface.
U.S. Patent Application Publication No. 2003/0020785, which is incorporated herein by reference in its entirety, discloses a laser abatable hydrophobic fluorine-containing polymer coating.
Conventionally, the aperture surface would be coated with fluoropolymer for anti-wetting purposes. Without the anti-wetting coating, the front face of printhead will flood with ink and the ink cannot be jetted out of the nozzle. The coating process is performed by evaporating fluoropolymer in a high vacuum chamber at elevated temperature. It is a batch process with printheads loaded and unloaded to and from the chamber for the coating, which is an expensive process.
Fluorinated compounds like fluoropolymers, in particularly poly(tetrafluoroethylene) (PTFE), are used extensively in low surface energy protective coatings to achieve wear resistant and environmental stability. For certain applications, where micro-particles of PTFE are required for mixing with other resins/binders, residues flake off and discharge of the microparticles after wear and tear can be a severe issue. Homogeneous coatings comprised of low surface energy moieties are more desirable. Unfortunately, in order to gain enough integrity, the low surface energy material must be compatible and best chemically linked with other components. Moreover, proper adhesion of the protective coatings to the base polymer, polyimide, is also critical.